Using code multiple access (CDMA=code division multiple access), a plurality of data streams can be transmitted simultaneously via a shared frequency band. In so doing, the symbols of the data streams to be transmitted are modulated by so-called interleaving codes.
The data streams transmitted simultaneously with various codes mutually interfere with each other, among other things: multipath propagation leads to overlapping of data symbols sent in succession (inter symbol interference, ISI). CDMA coding and multipath propagation are the cause of multiple access interference, (MAI). The interferences can be eliminated, for example, in the receiver if the pulse response of the channel is known there, as can be gathered from K. D. Kammeyer: “Nachrichtenübertragung”, 2nd edition, Information Technology series, Teubner, Stuttgart, 1996, and from A. Klein, G. K. Kaleh and P. W. Baier: “Zero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code-Division Multiple Access Channels”, IEEE Trans. Vehic. Tech., Vol. 45 (1996), 276-287. For example, the channel pulse response can be estimated in the receiver from a received reference signal.
However, interferences can also be eliminated in the transmitter if the channel pulse responses are known there. The channel pulse response is then no longer estimated in the receiver, but rather in the transmitter. Thus, the transmission of a reference signal from the receiver to the transmitter is necessary.
In particular, ISI and MAI can be eliminated in the transmitter by joint predistortion, (JP).
However, for forwarded data to be able to be detected by the receiver, the receiver must be synchronized to the transmitter. This is usually done by evaluation of the reference signal, transmitted at the same time, in the receiver (see, for example, B. Sklar, “Digital Communications”, Prentice Hall Int. Inc., Englewood Cliffs, 1988).
Transmission systems having interference elimination in the receiver and systems having elimination in the transmitter can also be combined with one another, as is known from Bosch: “Mixed Use of Joint Predistortion and Joint Detection in the UTRA TDD Mode”, ETSI Tdoc SMG2 UMTS-L1 205/98.
FIG. 3 shows a customary data burst structure in a CDMA data-transmission system, particularly a cellular CDMA data-transmission system, that operates in time division duplex (TDD).
In FIG. 3, B designates a data burst, t designates the time, DB1 designates a first data block, DB2 designates a second data block and RS designates a reference-signal data block.
FIG. 4 shows a schematic representation of the high-frequency (HF) processing in a known CDMA data-transmission system, and specifically, FIG. 4a in the reverse link and FIG. 4b in the forward link.
In FIG. 4, BS designates a base station, MS designates a mobile station, EHF designates a specific reception HF stage, SHF designates a specific transmission HF stage, RS designates a reverse link and VS designates a forward link.
In the reverse link, spread data signals and reference signals are transmitted, for example, via data bursts B shown in FIG. 3, to the base station for channel estimation.
In forward link VS, spread data signals are initially predistorted in a predistortion device VE, the result of the channel estimation in the reverse link being used, and then transmitted to the mobile station. Reference signals are transmitted without predistortion in the forward direction.
FIG. 5 shows a representation of the time sequence in the case of TDD operation with predistortion for the CDMA data-transmission system according to FIG. 4.
First of all, mobile station MS sends a data burst with a reference-signal data block RS to base station BS, which uses it for the channel estimation. Based on the result of the channel estimation, predistortion device VE carries out a predistortion and sends corresponding, predistorted, spread data signals in the form of data bursts B to the mobile station.
FIG. 6 shows an illustration of the shift in the moment of transmission in a known JP method given identical forward and reverse links, and specifically FIG. 6a in the reverse link and FIG. 6b in the forward link.
Generally stated, the known JP methods control the precise transmission moment of the data in forward link VS as a function of the channel estimations of reverse link RS. If the channel estimations contain very early channel pulse responses, transmission is carried out late and vice versa. If the transmission links are identical in the forward and reverse direction, this mechanism saves a synchronization adjustment in the receiver (here mobile station MS), since the signals reach the receiver at the same moment, regardless of the transmission-channel delay.
The time relationships of the channel estimation of reverse link RS used in the predistortion are shown in FIG. 6a, and the relationships of forward link VS are shown in FIG. 6b. 
As can be seen in FIG. 6a, mobile station MS sends a data burst B with a reference signal at a specific moment via reverse link RS to base station BS. In so doing, the reference signal experiences a delay by Δt (propagation delay) and a phase rotation by the angle Δφ.
From this, a channel estimation KS is created, which in turn is used for determining the predistortion parameters for predistortion VE. In the forward direction, the signals to be transmitted likewise experience a delay by the time span Δt, and a phase rotation by the angle Δφ. However, the predistortion offsets this by the derivative action Δt and the negative phase rotation Δφ, so that a synchronization of the data in data burst B is also achieved simultaneously.
The transmission links in the forward and reverse directions also include the HF parts of transmitter and receiver used in each case. They cause, inter alia, different delay times in forward and reverse links VS and RS, respectively. In addition, the delay times of forward and reverse links VS and RS differ in the case of variable channels (as, for example, in the case of receiver and transmitter moved relatively to each other), because the forward link and reverse link are used one after the other in time. Since only the delay time of the counter link is taken into account by the known JP method, the delay time of the transmit link is additionally taken into account by a synchronization mechanism in the receiver.
FIG. 7 shows an illustration of the shift in the moment of transmission in a known JP method, given delay differences in the forward and reverse links, and specifically, FIG. 7a in the reverse link and FIG. 7b in the forward link, when no synchronization is used.
According to FIG. 7a, the delay in the reverse link is Δtg and the phrase rotation in the reverse link is Δφg. Since in forward link VS, the delay and phase rotation are different, namely, Δts and Δφs, respectively, the data are asynchronous by the time interval −Δt=Δtg−Δts.
The shifts in moment of transmission of known JP methods are taken into account automatically by synchronization mechanisms based on reference signals, if filters are used for the predistortion and these filters are also used on the reference signals.
However, this has the disadvantage that no standard JP methods not based on filters can be used. Standard JP methods can supply better transmission qualities, however, than JP methods based on filters.
Customary synchronization mechanisms, which are based on reference signals that are not predistorted, do not take the shifts in transmission moment of JP methods into account.
FIG. 8 shows an illustration of the synchronization in a known JP method given delay differences in the forward and reverse links, and specifically, FIG. 8a in the reverse link and FIG. 8b, and FIG. 8c in the forward link.
It can be gathered from FIG. 8 that the data signals are sent shifted in time due to predistortion, but not the non-predistorted reference signal. Therefore, a synchronization RESY based on the non-predistorted reference signal does not, inter alia, find the synchronization moment desired for the data, but rather is asynchronous by Δts.